Imaging Latitude 7290 (what is legacy external and how do I avoid it)

I am testing the Latitude 7290. I usually switch from UEFI to legacy. Now legacy is external only. I am at a loss. I finally got it to image through lots of trial and error but now it only tries to boot to the network and not to the hard drive. When I go into BIOS I have 3 choices. UEFI no secure UEFI secure and Legacy External. None of which show the hard drive as a bootable device. Thoughts and ideas are welcome. My supervisor believes this to be a driver issue. I think it's a BIOS issue. Help!

Also how long does it usually take for the drivers of new models to show up in the Driver feed?

two things:
1. you are right with the driver issue.
You need to build a new KBE with the right driver:
Download the hdd driver from Dell, unzip it and add it to \\SDA\drivers\kbe_windows_x64, recache the drivers and as soon as it is done create a new KBE.
2. the drivers for new models are avaiable as soon as they are staged, this means, if Dell provides the driver pack it will be made avaiable for all appliances which are in support. This is a midlevel driver injection so it does not allow you to boot a kbe to a system when the kbe has not the right drivers in it (I hope there will be a future solution to inject the nessesary drivers automaticly to ta prebuilt KBE, but right now you need to build it by yourself)

So I used to change boot mode to legacy in order to get the option to boot from NIC. How do I do this now? I can't imagine that the drivers will help as they are not applied until after network booting. I have been through the bios and tried everything I can possibly think of. What am I missing?